This invention relates to a device for determining the concentration of at least one substance in organic tissue, including a hypodermic needle for insertion into the tissue, a pumping and suction unit provided in a housing, which unit is controlled by a microprocessor and is used for delivering a perfusion fluid and draining it after its partial equilibration with the tissue, and further including a sensing unit connected to the microprocessor for determining the concentration of the substance to be analyzed and one or more marker variables, as well as containers for the perfusion fluid, for a calibrating solution and at least one drug.